Architectural pattern for persistent web application design

ABSTRACT

A computer system operating a web application with offline capabilities, including a web browser, a local web application, a write queue maintained in the local memory and a persistent local database. When the web application needs to perform an operation on a data item, the web application issues a database request to determine if the data item is in the local database. If the data item is not in the local database, the web application issues the data request to the server system. If the data item is in the local database, the web application performs the operation on the data item stored in the database and writes the operation to the write queue along with the identifier of the data item. When a network connection exits between the client device and the server system, the web interface drains the write queue to the server system.

RELATED APPLICATIONS

This application relates to U.S. patent application Ser. No. 12/418,444, “System and Method for Reducing Startup Cost of a Software Application,” filed on Apr. 3, 2009, which is hereby incorporated by reference in its entirety.

This application relates to U.S. patent application Ser. No. 12/418,470, “Tracking Remote Browser Crashes via Cookies,” filed on Apr. 3, 2009, which is hereby incorporated by reference in its entirety.

This application relates to U.S. patent application Ser. No. 12/418,460, “Reduced Bandwidth Cache Coherency via Checksum Exchange,” filed on Apr. 3, 2009, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The disclosed embodiments relate generally to the field of client-server computer network systems, and in particular, to systems and methods for developing web applications with offline capabilities.

BACKGROUND

Web applications (e.g., applications that are primarily accessed through a web browser and receive data from a remote server) are becoming increasingly popular. Web applications are used for a wide variety of purposes including email, calendar, banking, word processing, and image editing applications. However, the widespread use of web applications has created a number of problems. For example there is often a delay between when a request is sent over a network and when a response is received, also known as network latency. This network latency reduces the responsiveness of web applications, because the web application must wait for data from a remote server before responding to requests to perform operations indicated by a user. This additional waiting time can be frustrating for the user. Additionally, when a network connection is not available, the web application is unable to receive data from the remote server and thus the functionality of the web application is limited. Similarly the inability to use an application due to a lack of a network connection can be frustrating for a user.

SUMMARY

Accordingly, the method and system disclosed herein reduces or eliminates the above described problems by providing local data structures for storing data items for use by the web application. Using the disclosed method and system, the negative effect of network latency and the lack of a network connection on user experience are reduced. In some embodiments, the data items stored in the local database are used by the web application in an offline mode. In these embodiments, the web application uses the locally stored data items to provide at least some of the functions commonly provided by the web application when there is a network connection (e.g., displaying email messages). In some embodiments these data items stored in the local database are used by the web application in an online mode to improve the responsiveness of the web application to user requests to perform operations (e.g., display operations) on the data items. For example, for email messages cached in the local database, the cached email messages can be displayed immediately when the user selects one of the cached messages for display, without waiting for a response to the request from a remote server.

According to a first aspect of the present invention, a client device has a computer processor and a computer memory; a web browser configured to execute on the client device; a local web application that is configured to operate within the web browser; a web interface configured to manage communications between the local web application and a server system associated with the web application; a write queue maintained in the memory; and a persistent local database maintained in the memory containing a plurality of data items, each being associated with an identifier that enables each respective data item to be uniquely identified on the client device and on the server system. When the web application on the client device needs to perform an operation on a data item, the web application issues a database request to determine if the data item is in the local database. If the data item is not in the local database, the web application issues the data request to the server system via the web interface. If the data item is in the local database and the operation involves updating the data item, the web application performs the operation on the data item stored in the database and writes information characterizing the operation to the write queue along with the identifier of the data item. When a network connection exits between the client device and the server system, the web interface drains the write queue to the server system, which can use this information to update entries in its database that were modified by the client operations. This is an example of a write-through cache. An embodiment of the present invention also implements a LRU (least recently used) cache replacement policy, in which space is freed for a new entry in the local database by deleting the least recently used entry.

In some embodiments, this software caching is analogized to hardware caching. By analogizing hardware caching strategies to software caching strategies, the performance of web applications that use software caching can be improved using techniques that are similar to those used to improve the performance of hardware caching. These caching techniques are often well-known and thoroughly tested. For example, the speed of cache access can be improved by inserting a high speed L1 cache between a processor (CPU) and a larger but slower L2 cache. Similarly, speed of access to data items for a web application can be improved by inserting an in-memory database between the web application and the persistent database. Other embodiments could use the document object model of the web browser as a L1 cache. As suggested by the kinds of caches found in hardware, software using the document object model as L1 cache could be write-back or write-through. A write-back cache of document object model fragments as an L1 cache would be appropriate if the web application implemented some user actions by directly modifying the document object model. Other features in the embodiment suggested by this analogy with hardware caching are prefetching of data items and load forwarding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the infrastructure of a client-server distributed system in accordance with some embodiments.

FIG. 2 is a block diagram illustrating the structure of an exemplary client device in accordance with some embodiments.

FIG. 3 is a block diagram illustrating the structure of an exemplary server system in accordance with some embodiments.

FIG. 4 is a block diagram illustrating the interrelation between components of the client-server distributed system in accordance with some embodiments.

FIGS. 5A-5C are block diagrams illustrating the startup process of a multi-thread software application in a client-server distributed system in accordance with some embodiments.

FIG. 6A is a block diagram illustrating data structures in a server system associated with web application in accordance with some embodiments.

FIG. 6B is a block diagram illustrating data structures in a client device associated with a persistent web application in accordance with some embodiments.

FIGS. 7A-7B depict a flow diagram illustrating client and server roles in a process for transferring data between a persistent web application on a client device and a server system in accordance with some embodiments.

FIGS. 8A-8D depict a flow diagram illustrating client and server roles in a process for synchronizing data between databases using a checksum exchange in accordance with some embodiments.

Like reference numerals refer to corresponding parts throughout the drawings.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a block diagram illustrating the infrastructure of a client-server distributed system according to some embodiments. The distributed system includes a plurality of client devices 102 and a plurality of server systems 106. These components are linked together through one or more communication networks 104 (e.g., the Internet, other wide area networks, local area networks, etc.) so that the various components can communicate with each other. In some embodiments, each of the server systems 106 is a single server. In other embodiments a server system 106 includes a plurality of servers such as a web interface (front end server) 108, one or more server applications 110 (which may be implemented on one or more servers) and one or more central databases 120 which are connected to each other through a local area network (LAN) and exchange information with the client devices 102 through a common interface (e.g., one or more server systems, also called front end servers). In embodiments having a plurality of server systems 106, the server systems 106 may be connected to each other through a local area network (LAN) or other communication network.

A client device 102 includes a client application such as a web browser 112. A user can use the web browser 112 to access one or more web applications 114 from the server systems 106. The web browser 112 and the web application 114 within the web browser 112 have access to data items stored in a local database 116 on the client 102. In some embodiments, accessing a web application 114 includes downloading a program from a server system 106 and storing data in a local database 116 for use by the web application 114. The client device 102 (sometimes called the “client device” or “client computer”) may be any computer or similar device that is capable of receiving data from and sending requests (e.g., web application data requests, search queries, information requests, login requests, etc.) to the server system 106. Examples of client devices include, without limitation, desktop computers, notebook computers, tablet computers, mobile devices such as mobile phones and personal digital assistants, and set-top boxes. In the present application, the term “web application” means virtually any interactive application that provides the user with access to content received from a server system 106. Requests from a client device 102 are conveyed to a respective server system 106 using the HTTP protocol, using http requests through a communication interface 118 or other similar network communication protocols.

A server system 106 includes at least a web interface (front end server) 108, a server application 110 and a central database 120. The web interface 108 parses requests from the client devices 102, fetches corresponding web applications provided by the server application 110 and returns the web applications to the requesting client device(s) 102. Depending upon their respective locations in the topology of the client-server system, the web interface is implemented on a separate (“front end server”) 108 and the server application is implemented on a separate (“back end server”) 110. In the present application, the terms “web interface” and “front end server” are used interchangeably. In some other embodiments, the front end server 108 and the back end server 110 are merged into one software application or one server system 106.

In some embodiments, the server systems 106 are deployed over multiple computers (represented by N application servers and N central databases in FIG. 1) so as to provide different types of services, such as email services, search engine services, map services, social networking services and the like. In some other embodiments, one individual type of service may also be distributed among multiple servers. For example, consider a system in which a server application 110-1 is a web-based email service (e.g., the Gmail email application). Data items such as: email messages, conversations (e.g., lists of email messages), thread lists (e.g., lists of conversations) and account information (e.g., user profiles, user preferences, and account history) may be stored in one or more central database(s) 120 that are accessible to the server application 110. Server applications 110 may be connected to one or more central databases 120. In some embodiments a single server application 110 may have access to a single central database 120 (such as where information stored in the central database is needed only by the single server applications), while in other embodiments multiple server applications 110 have access to a single central database 120 (such as where a small amount of information is used by a number of server applications 110 and the information is updated frequently), in other embodiments multiple server applications 110 may be connected to multiple central databases 120 (such as where a large amount of data is stored and needed by a large number of server applications 110).

Attention is now directed to FIG. 2, which is a block diagram illustrating a computer system (e.g., client device 102) in accordance with one embodiment of the present invention. The client device 102 typically includes one or more processing units (CPUs) 202, one or more network or other communications interfaces 204, memory 212, and one or more communication buses 214 for interconnecting these components. The client device 102 optionally may include a user interface 205 comprising a display device 206 and a keyboard/mouse 208 (or, in some embodiments a touch-sensitive surface 209). Memory 212 includes high-speed random access memory, such as DRAM, SRAM, DDR RAM or other random access solid state memory devices; and may include non-volatile memory, such as one or more magnetic disk storage devices, optical disk storage devices, flash memory devices, or other non-volatile solid state storage devices. Memory 212 may optionally include one or more storage devices remotely located from the CPU(s) 202. Memory 212, or alternately the non-volatile memory device(s) within memory 212, comprises a computer readable storage medium. In some embodiments, memory 212 or the computer readable storage medium of memory 212 stores the following programs, modules and data structures, or a subset thereof:

-   -   an operating system 216 that includes procedures for handling         various basic system services and for performing hardware         dependent tasks;     -   a communications module 218 that is used for connecting the         client device 102 to other computers via the one or more         communication interfaces 204 (wired or wireless) and one or more         communication networks, such as the Internet, other wide area         networks, local area networks, metropolitan area networks, and         so on;     -   a web browser 112, for receiving a user request for a web         application 114, rendering the requested web application 114 on         the display device 206 or other user interface device using a         user interface module 228, and storing data items in a local         database 116; in some embodiments, the web application 114,         local database 116 and the user interface module execute within         the web browser.         -   the one or more web applications 114 may include a web             application manager 219 for coordinating operations within             the web application 114, a write queue 220 for queuing             operations from the web application manager 219, an action             dispatcher 222 for writing to a local database 116 and             (optionally) a checksum calculator 224 for calculating             checksums on data items;         -   the local database 116 stores data items (e.g., 226-1,             226-2) received at the web application 114 from various             servers systems 106;         -   the user interface module 228 includes a view module 230 for             rendering the user interface and a controller module 232 for             detecting user interaction events and passing along user             interaction events to other modules; and     -   a response handler module 234 for responding to communications         from the server system by storing data items in the local         database 116 and communicating information to the web         application 114.

Although FIG. 2 shows a “client device 102” the client device described with reference to FIG. 2 is intended more as functional description of the various features which may be present in a client device 102 than as a structural schematic of the embodiments described herein. In practice, and as recognized by those of ordinary skill in the art, items shown separately could be combined and some items could be separated. For example, the client application (e.g., the web browser 112) may be integrated with the operating system 216 in some embodiments. In some embodiments, various functions of the client application (e.g., web browser 112) may be performed by two or more separate applications. In some embodiments, the local database 116, shown as part of the web browser 112, could be a local database accessible to a plurality of client applications on the client device.

Each of the above identified elements may be stored in one or more of the previously mentioned memory devices, and corresponds to a set of instructions for performing a function described above. The above identified modules or programs (i.e., sets of instructions) need not be implemented as separate software programs, procedures or modules, and thus various subsets of these modules may be combined or otherwise re-arranged in various embodiments. In some embodiments, memory 212 may store a subset of the modules and data structures identified above. Furthermore, memory 212 may store additional modules and data structures not described above.

Attention is now directed to FIG. 3, which is a block diagram illustrating a server system 106 in accordance with one embodiment of the present invention. The server system 106 typically includes one or more processing units (CPUs) 302, one or more network or other communications interfaces 304, memory 312, and one or more communication buses 314 for interconnecting these components. Memory 312 includes high-speed random access memory, such as DRAM, SRAM, DDR RAM or other random access solid state memory devices; and may include non-volatile memory, such as one or more magnetic disk storage devices, optical disk storage devices, flash memory devices, or other non-volatile solid state storage devices. Memory 312 may optionally include one or more storage devices remotely located from the CPU(s) 302. Memory 312, or alternately the non-volatile memory device(s) within memory 312, comprises a computer readable storage medium. In some embodiments, memory 312 or the computer readable storage medium of memory 312 stores the following programs, modules and data structures, or a subset thereof:

-   -   an operating system 316 that includes procedures for handling         various basic system services and for performing hardware         dependent tasks;     -   a communication module 318 that is used for connecting the         server system 106 to other computers via the one or more         communication interfaces 304 (wired or wireless) and one or more         communication networks, such as the Internet, other wide area         networks, local area networks, metropolitan area networks, and         so on;     -   a response handler module 320 that passes communications between         the server system 106 and client devices 102, the response         handler module can be used in conjunction with or instead of a         separate front end server (108 in FIG. 1);     -   a server application 110 that prepares responses to requests         that are received from the client device 102, in some         embodiments the server application 110 sends the web application         114 to the client device 102;     -   a central database 120 for storing data items (e.g., 226-1,         226-2) associated with the server application 110 and/or the web         application 114;     -   a checksum calculator (optional) 328 for calculating a checksum         on a data item; and     -   a checksum comparer (optional) 330 for comparing a checksum         received from a web application with a checksum computed by the         server system 106.

Although FIG. 3 shows a single server system 106, the server described in with reference to FIG. 3 is intended more as functional description of the various features which may be present in a server system than as a structural schematic of the embodiments described herein. In practice, and as recognized by those of ordinary skill in the art, items shown separately could be combined and some items could be separated. For example, some items shown separately in FIG. 3 could be implemented on single server, and single items could be implemented by one or more servers. The actual number and types of servers used to implement an server system 106 (FIG. 1) and how features are allocated among them will vary from one implementation to another, and may depend in part on the amount of data traffic that the system must handle during peak usage periods as well as during average usage periods.

Each of the above identified elements may be stored in one or more of the previously mentioned memory devices, and corresponds to a set of instructions for performing a function described above. The above identified modules or programs (i.e., sets of instructions) need not be implemented as separate software programs, procedures or modules, and thus various subsets of these modules may be combined or otherwise re-arranged in various embodiments. In some embodiments, memory 312 may store a subset of the modules and data structures identified above. Furthermore, memory 312 may store additional modules and data structures not described above.

Attention is now directed to FIG. 4, which illustrates the interrelation between components of a client-server distributed system in accordance with some embodiments. As shown in FIG. 4, the user interface module 228 includes a view module 230 for rendering the user interface (e.g., on the display 206 in FIG. 2), and a controller module 232 for detecting user interaction events (e.g., mouse clicks or keyboard inputs from keyboard/mouse 208 or touch sensitive surface 209 in FIG. 2) and passing the user interaction events to the web application manager 219. In some embodiments the controller module 232 is implemented as a set of event handlers 402 (e.g., click/touch event handlers), which handle clicks, touches and other user input. In some embodiments, initially all event handlers display activity, and send a request to the web application manager 219 (e.g., in some embodiments, a JavaScript in-memory model), which sends a model event back to the view module 230. In the view module 230, a model event handler 406 passes the event off to a user interface renderer 408 (e.g., a HTML renderer) and a document object model 410 or other application programmer interface for manipulating documents. The user interface renderer 408 sends rendered user interface elements to the document object model 410, which constructs a new model, displays the model (e.g., to the user) and passes the new model off to the controller module 232 to wait for additional events detected by the event handlers 402.

In some embodiments, when the web application manager 219 is configured to detect an event from the event handler 402 indicating an operation to be performed on one or more of the data items in the local database 116, the web application manager 219 sends the operation to the write queue 220. In some embodiments, operations 412-1, 412-2, 412-3, 412-4, 412-5, 412-6 are stored in the write queue 220 in the order in which they arrived at the write queue 220 (i.e., in these embodiments the write queue is a chronological record of requests from the web application manager 219). The web application manager maintains a list of callbacks into the user interface module 228, one for each operation (e.g., “Operation 1” 412-1) in the write queue, a flag indicating whether the callback has been called or not, and a counter for failed attempts to satisfy the operation (e.g., “Operation 1” 412-1). In some embodiments, the action dispatcher 222 is configured to send operations (e.g., “Operation 1” 412-1) from the write queue 220 to the local database 116, and to the communications module 218.

In some embodiments the action dispatcher 222 is configured to send operations to a checksum calculator. The checksum calculator computes the checksum of one or more data items stored in the local database 116. In some embodiments, the checksum is stored in the local database 116 along with the data item. If there was a previous checksum for the data item stored in the local database 116, the computed checksum replaces the stored checksum. In some embodiments, checksums are computed using the whole data item. A checksum can be computed in any of the standard mechanisms known in the art (e.g., the MD5 checksum algorithm (IETF RFC 1321) or SHA-1 (FIPS PUB 180-1) algorithm). In some embodiments, discussed in greater detail below with reference to FIGS. 8A-8D, checksums for a data item are used to determine whether the data item has been updated.

In some embodiments, the communications module 218 is configured to transmit the operations (e.g., “Operation 1” 412-1) received from the action dispatcher 222 to the response handler module 320 in the server system. In some embodiments the response handler module 320 is configured to pass off the operation to the server application 110. The server application 110 is, in some embodiments, configured to retrieve one or more data items from a central database 120 in response to receiving the request to perform the operation (e.g., using a unique identifier associated with the data item that was included in the request). In some embodiments, the server application 110 is configured to return the results (e.g., a data item and/or a checksum of a data item) of the operation to the response handler 320, which is configured to transmit the results to the client device 102. In some embodiments when the local database 116 is not synchronized (i.e., is not coherent) with the central database 120, the response includes a replacement data item or a component of a data item needed to bring the local database 116 back into coherence with the central database 116. In some embodiments, when the local database 116 is synchronized (i.e., coherent) with the central database 120, the response indicates that the data items in the local database 116 are up to date (e.g., the response does not include any data items or components of data items).

In some embodiments, the communications module 218 on the client device 102 passes off the results from the server application 110 to a response handler module 234 on the client device. In some embodiments the results of the operation are stored in the local database 116 and/or returned to the web application manager 219. In some embodiments the operation is a display operation and the data item is displayed to the user.

For example, consider a scenario where a user is viewing an email application on a mobile device and selects the header of the email to view the email. In this example, the request to view the email message on a mobile device is sent to the write queue 220 by the web application manager 219. If the action dispatcher 222 finds the email in the local database 116, then the email is sent to the web application manager 219 for display to the user. If the action dispatcher 222 does not find the email in the local database 116, then the request for the message is sent to a server system 106 with an email database 120 and the email from the central database 120 returned to the client device 102. In this embodiment, the email message is returned to the response handler 234, which stores the email in the local database 116 and passes it off to the web application manager 219 for display to the user.

Attention is now directed to FIGS. 5A-5C, which illustrate the startup process of a multi-thread software application in a client-server distributed system in accordance with some embodiments.

To reduce the startup cost at the client 102, the multi-thread software application divides the process into multiple stages as shown in FIGS. 5A-5C, respectively. At the initialization stage 510, the software application first launches a main application on a main thread 502. In some embodiments, the software application is the web-based email service on a portable device such as a mobile phone. The main application on the main thread is responsible for handling user interactions with the user interface of the service. In some embodiments, the main application is single-threaded. In some other embodiments, the main application is multi-threaded.

As shown in FIG. 5A, after the initialization, there is a synchronous connection 506 between the main thread 502 and a local database 118. The local database 118 caches a predefined amount of data associated with the email service. A more detailed description of the local database is provided below in connection with FIG. 6B. Through the synchronous connection 506, the main application at the main thread 502 is able to perform operations on the local database 118 in a synchronous manner such as retrieving data records from the local database 118 and updating data records in the local database 118.

There is also an asynchronous connection 514 between the main thread 502 and the server application 322 at a remote server system 106. As part of a web-based software application, all the email messages rendered by the main application at the client 102 ultimately come from the server application 322, which manages the email messages in a central database 120 at the server side. A more detailed description of the server application 322 and the central database 120 is provided below in connection with FIGS. 6A and 7A-7B. As will be explained below, the main application uses the asynchronous connection 514 to retrieve data from the central database 120 if the data is not present in the local database 118.

In some embodiments, the initialization of the main application at the main thread 502 includes displaying a user interface at the client 102, retrieving a predefined amount of data from the local database 118, and populating the user interface with the retrieved data. For example, during the initialization of a web-based email service, the main application on the main thread 502 uses the synchronous connection 506 to retrieve the email messages associated with the inbox folder of a user's email account from the local database 118 because it is assumed that an average user of the email service is most interested in first viewing the unread messages in the inbox folder directed to the user. After retrieving the messages from the local database 118 through the synchronous connection 506, the main application uses the retrieved messages to populate the user interface associated with the email service to allow the user of the client 102 to interact with at least the messages in the inbox folder. Although the initialization of the main application as shown in FIG. 5A may not conclude the startup of the entire email service, being able to access the unread messages in the inbox folder as quickly as possible can significantly improve an average user's experience with the email service.

Besides populating the inbox folder, the startup process of the email service may include other tasks such as fetching email messages associated with other folders from the local database 118 and synchronizing the local database 118 with the central database 120. As a result, the completion of the entire startup process may involve a significant amount of data transmission between different components in the client 102 and between the client 102 and the server 106. If the main thread 502 is also responsible for handling these tasks, it may not be able to bring up the user interface and get ready to receive the user instructions in a timely fashion.

Thus, in some embodiments, the initialization stage 510, which gives the user an impression that the email service is ready, is followed by the second stage 520, i.e., initializing an assistant process on a secondary thread 504. By deferring the start of the secondary thread 504, the latency caused by having the main thread 502 responsible for the entire startup process is avoided or at least significantly reduced.

As shown in FIG. 5B, the main thread 502 spawns the secondary thread 504 after the main application at the main thread 502 concludes a transaction between the main thread 502 and the local database 116, e.g., populating the inbox folder of the email service. Note that the assistant process at the secondary thread 504 is generated to substitute the main application at the main thread 502 for communicating with the local database 118 and completing the remaining tasks of starting the web-based email service application. There is also a synchronous connection 512 between the secondary thread 504 and the local database 116. Through this connection 512, the assistant process can perform the same operations on the local database 118 that the main application can perform through the connection 506. In some embodiments, the assistant process is configured to complete the remaining tasks of initializing the web-based application left by the main application. In some other embodiments, the main application completes all the tasks associated with initializing the web-based application. Thus, the assistant process is primarily responsible for replacing the role of interacting with the local database 118 that was played by the main thread 502.

As shown in FIG. 5C, there is an asynchronous connection 508 between the main thread 502 and the secondary thread 504. With the establishment of the synchronous connection 512, the main application no longer needs to perform operations on the local database 116 directly through the synchronous connection 506 as it did during the initialization stage 510. Rather, the entire application now transits into the stage 530 of steady operation. During the steady operation, the main application communicates with the assistant process at the secondary thread 504 through the asynchronous connection 508. As a result, in some embodiments, the synchronous connection 506 between the main thread 502 and the local database 116 is terminated or destroyed (as indicated by the dashed line in FIG. 5C). In some other embodiments, the synchronous connection 506 still exists but is deactivated.

Attention is now directed to FIG. 6A, which illustrates data structures in a server system associated with a web application in accordance with some embodiments. In some embodiments, the central database 120 includes a server directory 602. The server directory 602 is a lookup table that associates a unique identifier (e.g., “Unique ID” 604-1) with the status (e.g., “Status” 606-1) of the data item (e.g., Data Item 1 226-1) and a reference (e.g., “Reference 1” 608-1) to the data item.

In some embodiments, the unique identifier (e.g., “Unique ID” 604-N) includes a data item type 609 and a data item identifier 610. In some embodiments, the data item type 609 indicates the type of data item referenced by the unique identifier. For example, when the data item is a conversation (e.g., a list of messages), the data item type 609 includes “conversation” or a corresponding alphanumeric tag, and when the data item is a thread list (e.g., a list of conversations) the data item type 609 includes “thread list” or a corresponding alphanumeric tag. Alternatively, when the data item is a thread list (e.g., a list of conversations) the data item type 609 includes the name of the thread list. For example, when the thread list is an email inbox the data item type includes the alphanumeric tag “inbox,” and when the thread list includes a plurality of conversations associated with the label “work,” the data item type includes the alphanumeric tag “work.” In some embodiments the data item type identifier may be a combination of data item type identifiers.

In some embodiments, the data item identifier is an identifier that uniquely identifies the data item within a subset of data items that is of a particular type (e.g., where the data item type 609 is “conversation” or where the data item type 609 is “message”). For example, in some embodiments, the data item identifier is a conversation ID (e.g., 620) or a message ID. In embodiment where the data item is a conversation (e.g., a list of messages), the data item identifier is the message identifier of the first message in the list of messages.

In some embodiments, the status (e.g., 606-1) of a data item (e.g., 226-1) is indicative of the last time that the data item was updated (e.g., replaced with a new data item), modified or accessed (e.g., viewed, copied, sent, etc.). The status is updated when an operation is performed on the data item.

In some embodiments the reference to a data item is a reference to a relation table 611 that includes a reference from the data item to each component of the data item. In some embodiments, the data item has multiple components. For example, reference 1 608-1 in the server directory 602 refers to two entries in the relation table 611 for data item 1 226-1: thread 1 612-1 and thread 2 612-2. (This situation arises because these two conversations/threads are each associated with the thread list identified by the reference 608-1). In some embodiments, the data item has a single component. For example, reference 2 608-2 refers to a single entry in the relation table 611 for data item 2 226-2: thread 2 612-2. In some embodiments, a plurality of distinct data items include a shared component. For example, data item 1 226-1 and data item 2 226-2 are each related to thread 2 612-2 in the relation table 611. In other words, a single conversation (e.g., a list of messages) may be included in multiple thread lists. For example, in an embodiment where the web application is a web-based email application and email messages are arranged into conversations (e.g., ordered lists of email messages), one or more of the conversations may include a plurality of labels, where each label is associated with a thread list (e.g., a conversation including the “inbox” label and the “work” label would be associated with both the “inbox” thread list and the “work” thread list).

In some embodiments the components are messages. As described above, the data item may include a single component or multiple components. For example, reference 3 608-3 (which is a reference for a conversation) refers to three entries in the relation table 611 for data item 3 226-3: message 1 614-1, message 2 614-2 and message 3 614-3, while reference N 608-N refers to a single entry in the relation table data item-N 226-N (e.g., message Q 614-Q).

In some embodiments, when a relation in the relation table is to a thread (e.g., thread 1 612-1), the relation refers to a table of threads 616, including a plurality of threads (e.g., thread 1 612-1, thread 2 612-2, thread P 612-P). In the table of threads 616, each thread is associated with a conversation identifier 620 and conversation information 622 about the thread. The conversation identifier 620 is an identifier that is used by the server system to identify messages that are associated with the conversation. In some embodiments the conversation information 622 includes the subject of the messages included in the conversation, a short “snippet” summarizing the most recent message in the conversation and/or the list of messages.

In some embodiments, when a relation in the relation table is to a message (e.g., message 1 614-1), the relation refers to a table of messages 618, including a plurality of messages (e.g., message 1 614-1, message 2 614-2, message 3 614-3, message Q 614-Q). In the table of messages 618, each message is associated with message information 624 about the message. In some embodiments the message information 624 includes the text of a message including any attachments.

Attention is now directed to FIG. 6B, which illustrates data associated with a web application in a client device in accordance with some embodiments. In some embodiments, the local database 116 includes a client directory 652. The client directory 652 is a lookup table that associates a unique identifier (e.g., “Unique ID” 604-1) with the status (e.g., “Status” 656-1) of the data item (e.g., Data Item 1 226-1) and a reference to the data item (e.g., “Reference 1” 660-1). In some embodiments, where checksums (e.g., 658-1, 658-2, 658-3, 658-M) are used to maintain cache coherence, a checksum for each data item (e.g., 226-1, a checksum (e.g., 658-1) is stored in the local database 116 and associated with the data item. In the embodiment illustrated in FIG. 6B, the checksum (e.g., 658-1) is stored in the client directory 652. However, it should be understood that the checksums could be stored in another part of the local database 116.

In some embodiments, the unique identifier (e.g., “Unique ID” 604-N) includes a data item type 609 and a data item identifier 610, as discussed in greater detail above with reference to FIG. 6A. In some embodiments a unique identifier on the server (e.g., “Unique ID” 604-1 in FIG. 6A) is identical to a unique identifier on the client (e.g., “Unique ID” 604-1 in FIG. 6B) and indicates that both unique identifiers are associated with the same data item.

In some embodiments, the status (e.g., 656-1) of a data item (e.g., 226-1) is indicative of the last time that the data item was updated (e.g., replaced with a new data item), modified or accessed (e.g., viewed, copied, sent, etc.). The status is updated when an operation is performed on the data item. In some embodiments the status of a data item on the server (e.g., 606-2 in FIG. 6A) and the status of the same data item on the client (e.g., 656-2 in FIG. 6B) are identical (e.g., when the local database and the central database are synchronized). In some embodiments the status of the data item on the server (e.g., 606-3 in FIG. 6A) and the status of the same data item on the client (e.g., 656-3 in FIG. 6B) are different (e.g., when the local database and the central database are not synchronized).

In some embodiments the reference to a data item is a reference to a relation table 666 that includes a relation between the data item and each component of the data item. In some embodiments, the data item has multiple components. For example, the reference 1 660-1 in the client directory 652 refers to two entries in the relation table 666 for data item 1 226-1: thread 1 668-1 and thread 2 668-2. (This situation arises because these two conversations/threads are each associated with the thread list identified by the reference 608-1). In some embodiments, the data item has a single component (for example, the reference 2 660-2 refers to a single entry in the relation table 666 for data item 2 226-2: thread 2 668-2.) In some embodiments, a plurality of distinct data items include a shared component. For example, data item 1 and data item 2 each contain a relation to thread 2 668-2. In other words, a single conversation (e.g., a list of messages) may be included in multiple thread lists. For example, in an embodiment where the web application is a mobile email application and email messages are arranged into conversations, one or more of the conversations may include a plurality of labels, where each label is associated with a thread list (e.g., a conversation including the “inbox” label and the “work” label would be associated both with the “inbox” thread list and the “work” thread list).

In some embodiments the components are messages. As described above, the data item may include a single component or multiple components. For example, reference 3 660-3 (which is a reference to a conversation) refers to two entries in the relation table 666 for data item 3 226-3: message 1 670-1, and message 2 670-2, while reference M 660-M refers to a single entry in the relation table data item M 226-M (e.g., “Message S” 670-S).

In some embodiments, when a reference in the reference table is to a thread (e.g., thread 1 668-1), the reference refers to a table of threads 672, including a plurality of thread (e.g., thread 1 668-1, thread 2 668-2, thread R 668-R). In the table of threads 672, each thread is associated with a conversation identifier 676 and conversation information 678 about the thread. The conversation identifier 676 is an identifier that is used by the client device to identify messages that are associated with the conversation (e.g., a list of messages). In some embodiments the conversation information 678 includes the subject of the messages included in the conversation, a short “snippet” summarizing the most recent message in the conversation and/or the list of messages.

In some embodiments, when a reference in the reference table is to a message (e.g., message 1 670-1), the reference refers to a table of messages 674, including a plurality of messages (e.g., message 1 670-1, message 2 670-2, message S 670-S). In the table of messages 674, each message is associated with message information 680 about the message. In some embodiments the message information 680 includes the text of a message including any attachments.

In some embodiments, as illustrated in FIGS. 6A and 6B, the central database 120 in FIG. 6A and the local database 116 in FIG. 6B have similar data structures. However, the data stored in these data structures is not always identical. In some embodiments, the central database 120 in FIG. 6A and the local database 116 in FIG. 6B have different data items, messages and threads stored in their respective data structures. In some embodiments the data items, messages and threads in the local database 116 in FIG. 6B and the central database 120 in FIG. 6A have been synchronized (e.g., made coherent) and are identical. In accordance with some embodiments, methods are disclosed for detecting when the local database 116 in FIG. 6B and the central database 120 in FIG. 6A are not synchronized and for synchronizing the databases, as described in greater detail below with reference to FIGS. 8A-8D.

For example, in FIG. 6A, the central database has N data items, P threads and Q messages (where N, P and Q all represent integers greater than 0), while in FIG. 6B, the local database 116 has M data items, R threads and S messages (where M, R and S all represent integers greater than 0). In some embodiments N=M, P=R, Q=S, and the data structures in the central database 120 in FIG. 6A store identical information to the data structures in the local database 116 in FIG. 6B. However, in some embodiments the central database 120 in FIG. 6A contains additional data that is not contained in the local database 116 in FIG. 6B and/or the local database 116 in FIG. 6B contains additional data that is not contained in the central database 120 in FIG. 6A (e.g., the central database is associated with a web-based email server, which receives one or more new email messages while the client device is not connected to the email server, and thus the central database contains one or more new email messages that are not contained in the local database).

Additionally, it should be noted that, in some embodiments, checksums 658-1 are stored in the local database 116 in FIG. 6B, while they are not stored in the central database 120 in FIG. 6A. Rather, as described in greater detail below with reference to FIGS. 8A-8D, checksums are computed by the server system on-the-fly (e.g. on an “as needed” basis) and thus are not stored in the central database 120 in FIG. 6A.

Although exemplary data structures have been described herein for illustrative purposes, it should be understood that alternative data structures could be used in their place to store the data items and associated information about the data items described above. In some embodiments multiple servers containing multiple databases are used to store the data items on the server system, and different data structures specially adapted to such distributed systems may be used. In one such alternate embodiment, data structures such as those described in U.S. patent publication no. 2005/0222985 A1 (which is hereby incorporated by reference in its entirety) are used to store the data items, conversations and related information which are described as being stored in central database 120. For example, in a web-based email application, emails, conversations and other related information may be stored in data structures that are specially adapted to be distributed among a plurality of servers and databases to improve access speed and fault tolerance.

Attention is now directed to FIGS. 7A-7B, which illustrate client and server roles in a process for transferring data between a persistent web application on a client and a server system in accordance with some embodiments. The client (102 in FIG. 2) has a client application (e.g., web browser 112 in FIG. 2) including a local web application (e.g., 114 in FIG. 2). In some embodiments the local web application (e.g., 114 in FIG. 2) has both online and offline modes. For example, when the web application is a web-based email program, the program may store some of the frequently used email messages (e.g., data item 226-1 in FIG. 2) in a local database (e.g., 116 in FIG. 2).

The process begins when the web application needs access (702) to a data item such as an electronic message (e.g. an email), a conversation (e.g., a list of electronic messages), or a thread list (e.g., a list of conversations). In one example, the web application receives a request from a user to display the data item, send the data item, edit the data item, or perform some other operation on the data item. The client checks to see if the data item is in the local database. If the data item is (704) found in the local database, the data item is retrieved (706) from the local database and the web application receives (708) the data item.

In some embodiments, the web application is a web-based email application (e.g., GMail) with offline capabilities (e.g., is a persistent web-based application). For example, when the client has access to a network connection, the web application behaves just like a normal web-based email interface, where the user logs into a web page and checks email messages and performs other operations. However, in some embodiments, the web-based email program has offline capabilities because it has access to a local database that is used to store some of the email messages that are most likely to be accessed by the web application and other local application data. Then, when the client does not have access to a network connection and the user attempts to access the web-based email program through the web browser, the web application still allows the user to access the emails stored in the local database, and perform operations on the emails stored in the local database.

In some embodiments (e.g., where network latency is large) it is desirable to store some data items in a local database so as to reduce the delay between receiving an input from a user and performing an operation indicated by the input on (e.g., displaying) one or more data items. Thus, a local database can act as a cache for data items so that instructions from the web application can be performed on the data items in the local database before receiving a reply from the server system. This structure is analogous to a hardware cache associated with a microprocessor that is used to speed up processing operations in a hardware system by caching data for use by the processor and thus reducing the effects of communication latency between requests for data and receipt of the requested data by the microprocessor.

In some embodiments, the document object model of the web browser is used as a L1 cache. As suggested by the kinds of caches found in hardware, software using the document object model as L1 cache could be write-back or write-through. In some embodiments, a write-back cache of document object model fragments as an L1 cache is appropriate when the web application implements some user actions by directly modifying the document object model. Other features in the embodiment suggested by its analogy with hardware caching, in accordance with some embodiments, are prefetching of data items and load forwarding (i.e., where responses from the server are forwarded directly to the web browser 112 for display on display 206 before being written to local database 116).

In some embodiments, the write queue 220 is stored in non-volatile memory, so that even if the web application crashes or the web browser running the web application crashes, any operations written to the write queue by the web application up to that point are preserved.

In some embodiments, a requested data item may not be in the local database because it has never been requested by the client, or it may not be in the local database, because it has been removed from the local database due to a cache replacement policy. In some embodiments, when the system detects that the local database (e.g., cache) is full, at least some of the data items in the local database (e.g., cache) are marked for deletion in accordance with a cache replacement policy, which can be one of a number of options.

In some embodiments the cache replacement policy is a least recently used (LRU) policy in which the data items that were least recently used are deleted from the local database first. In some embodiments the cache replacement policy is a least frequently used policy in which the data items that are accessed least often are deleted from the local database first. In some embodiments, the cache replacement policy is determined by some other factor including which data items are the oldest or have the largest file size. In some embodiments, the cache replacement policy is random, in which data items are randomly (i.e., without regard for any criteria associated with the data item) deleted from the cache. In some embodiments the cache replacement policy is based on a combination of factors that determine which data items are most-likely to not be needed by the web application (e.g., knowing that email with certain characteristics might not be read more than once). In one embodiment a replacement priority is computed at the server for each data item and stored in the local database, and data items in the local database are marked for deletion in descending order by replacement priority. Additionally, it should be understood that any cache replacement strategy that is used in hardware caches could be used to determine which data items in the local database are marked for deletion. In some embodiments, the client periodically removes all data items that are marked for deletion from the local database. Teaching in the prior art as to application of these replacement strategies is relevant to different embodiments of the present invention.

If the data item is not found (710) in the local database and the client does not detect (712) a network connection, and error is returned (714) to the web application. In some embodiments an error is also returned (e.g., displayed) to the user of the web application.

If a network connection is detected (716), then a request is sent to the server system including a unique identifier (e.g., 604-1 in FIG. 6B) using a communication interface (e.g., web interface). The server system receives (718) the request and, in some embodiments, uses the unique identifier (e.g., 604-1 in FIG. 6B) to look up a data item in a central database (e.g., 120 in FIG. 6A) by matching the received unique identifier (e.g., with a unique identifier (e.g., 604-1 in FIG. 6A) in a server directory (e.g., 602 in FIG. 6A). The server system retrieves (719) the data item (and in some embodiments, additional information associated with the data item) from the central database, and sends the data item (and any associated information) to the client.

In some embodiments, the request is specified by the user such as a request to display an uncompressed representation (sometimes called an expanded representation) of the data item after detecting a selection of a compressed representation of the data item. In one example, an email inbox is displayed, the email inbox containing header information (e.g., the sender and subject) for a plurality of email messages or groups of email messages (e.g., one or more related email messages may be grouped together under one header as a single conversation). In this example, when the user selects the header of one email message or conversation and the web application displays the entire email message (when the header is a header for a single message) or a list of at least some of the email messages in the conversation (when the header is a header for a conversation of email messages).

In some embodiments, the request is automatically generated by the web application. In some embodiments the automatically generated request is to preload the data items that are most likely to be accessed by the user. In one example, an inbox is displayed containing header information (e.g., the sender and subject) for a plurality of email messages or groups of email messages (e.g., one or more related email messages may be grouped together under one header as a single conversation). In this example, the web application automatically generates a request for the data item associated with each of the headers in the current view of the inbox. It should be understood that the current view of the inbox could be any one of: all of the headers in the inbox (e.g., all of the messages/conversations in the inbox are requested), all of the headers that are included on a current page of the inbox (e.g., all of the messages/conversations that the user can scroll to on the screen are requested), and all of the headers that are currently displayed on the display (e.g., only the messages/conversations associated with the headers that are currently viewable by the user without scrolling are requested).

The client receives (708) the data item and performs (720) an operation on the data item. In some embodiments the operation does not update the data item (e.g., an email is displayed to a user). In some embodiments the operation does update the data item (e.g., the operation is a modification operation such as adding to or deleting an email message from a conversation of email messages). If the operation does not (722) update the data item, then the web application process ends (723). If the operation does (724) update the data item, then the web application places (726) a request to perform the operation in the write queue (e.g., 220 in FIG. 4). In some embodiments, the operations in the write queue include the unique identifiers of any data items associated with the operation (e.g., if the operation is to add a message to a respective conversation, then the operation includes the unique identifier of the respective conversation). In some embodiments, the server system uses this unique identifier to identify the data item stored in the central database on which the operation is to be performed (e.g., the server system identifies the respective conversation stored in the central database and adds the message to it).

In some embodiments, after placing an operation in the write queue, the web application checks (728) the network connection status. If the client is not (732) connected to the network, then the client device waits for a timeout (734) period and the rechecks (728) the network connection status. In some embodiments, the web application periodically checks the network connection status based whether or not an operation has been added to the write queue (e.g., every 5 minutes).

When the web application detects that the client device is (736) connected to the network, the write queue maintained in client memory is drained to the connected server. In some embodiments, it is important that the write queue is maintained in non-volatile memory so that user actions that have been made but not yet sent to the server system (e.g., because there is no network connection) are saved if the web browser crashes. In some embodiments, the write queue is drained in the order in which operations were added to the write queue (e.g., the oldest operation is sent to the server system first, followed by the second oldest, etc.). In some embodiments, it is important that the actions in the write queue are implemented in the order added to the queue, so as to ensure that when there are multiple operations that modify the same data item, the operations are performed in the correct order. (e.g., a first operation applies the label “work” to a respective message/conversation and a second operation marks all messages/conversations associated with the “work” label with a flag that indicates that the message/conversation has been read by the user).

In some embodiments, the server system receives (740) the request from the web application and responds (742) to the request (e.g., by performing the operation indicated in the request on one or more data items indicated by the unique identifier(s) associated with the request). In some embodiments, any data items that were modified by the operation are stored (744) in the central database on the server system. In some embodiments, the response to the request includes sending a response to the web application on the client including any modified data items. The client receives (746) the response and stores (748) any modified data items in the local database. In some embodiments, the client makes the modifications to the data items in the local database as the operations are drained from the write queue instead of (or before) receiving any response from the server to the request to perform the operations.

Attention is now directed to FIGS. 8A-8D, which illustrate client and server roles in a process for synchronizing data between databases using a checksum exchange in accordance with some embodiments. In some embodiments the client includes a local database that needs to be synchronized (e.g., made coherent) with a central database on a server system or other distinct computing device that is distinct from the client device. In one example, the web application is a web-based email application that stores email messages, lists of related email messages (e.g., conversations), and lists of related conversations (e.g., thread lists). In this example, the web-based email application provides a user with access to the messages/conversations in an email account, and it is desirable to keep the web application (e.g., an application for accessing web-based email) synchronized with a server application (e.g., an email server that manages all of the emails/conversations associated with the email account).

The process begins when the web application needs access (802) to a data item such as an electronic message (e.g. an email), a conversation (e.g., a list of electronic messages), or a thread list (e.g., a list of conversations). In one example, the web application receives a request from a user to display the data item, send the data item, edit the data item, or perform some other operation on the data item. The client checks to see if the data item is in the local database. If the data item is (804) found in the local database, the data item is retrieved (806) from the local database and the web application receives (808) the data item. In some embodiments, the data item retrieved from the local database includes a first checksum that was stored in the local database previously, as discussed in greater detail below.

In some embodiments, the web application is a web-based email application (e.g., GMail) with offline capabilities. For example, when the client has access to a network connection, the web application behaves just like a normal web-based email interface, where the user logs into a web page and checks email messages and performs other operations. However, in some embodiments, the web-based email application has offline capabilities, enabled by providing the application with access to a local database, which stores some of the email messages that are most likely to be accessed by the web application. Then, when the client does not have access to a network connection and the user attempts to access the web-based email program through the web browser, the web application (e.g., web-based email application) still allows the user to access the emails stored in the local database, and perform operations on the emails stored in the local database.

In some embodiments, the client operates in an offline mode, including accessing a data item stored in the local database, detecting an operation performed on the data item, writing information characterizing the operation to the write queue along with the identifier of the data item, computing a checksum of the updated data item and storing the computed checksum in the local database, as discussed in greater detail above with reference to FIGS. 7A-7B. In some embodiments, when a network connection is detected between the web application and the server system, the client drains the write queue to the server system (e.g., a web-based email server or a server system), where each request sent to the server system includes the operation, a unique identifier of one or more data items modified by the operation, and a checksum for each of the one or more data items.

The local database is stored in non-volatile memory, so that even if the web application crashes, any operations written to the write queue by the web application up to that point are preserved.

A data item may not be in the local database because it has never been requested by the client, or it may not be in the local database, because it has been removed from the local database through the implementation of a cache replacement policy. In some embodiments, when the system detects that the local database (e.g., cache) is full, at least some of the data items in the local database (e.g., cache) are marked for deletion in accordance with a cache replacement policy, as described in greater detail above with reference to FIGS. 7A-7B.

If the data item is not found (810) in the local database and the client does not detect (812) a network connection, and error is returned (814) to the web application. In some embodiments an error is also returned (e.g., displayed) to the user of the web application.

If a network connection is detected (816), then a request is sent to the server system including a unique identifier (e.g., 604-1 in FIG. 6B) using a communication interface (e.g., web interface). The server system receives (818) the request and, in some embodiments, uses the unique identifier (e.g., 604-1 in FIG. 6B) to look up a data item in a central database (e.g., 120 in FIG. 6A) by matching the received unique identifier (e.g., with a unique identifier (e.g., 604-1 in FIG. 6A) in a server directory (e.g., 602 in FIG. 6A). The server system retrieves (820) the data item (and in some embodiments, additional information associated with the data item) from the central database, and sends the data item (and any associated information) to the client.

In some embodiments the server system computes (820) a first checksum on the data item, and sends the data item and the first checksum associated with the data item to the client. The client receives (808) the data item and the first checksum in the local database and performs (826) an operation on the data item (e.g., displays the data item). In some embodiments, performing an operation on the data item includes modifying the data item, and the client computes a checksum for the modified data item and replaces the first checksum that is stored in the local database with the client-computed checksum.

In some embodiments, at a later point in time, the web application needs (828) access to the data item for a second time. The client checks to determine whether the data item is in the local database. Even though the data item was previously requested from the server system and stored in the local database, the data item may no longer be stored in the local database. For example, if the data item has been removed from the local database through the implementation of a cache replacement policy, the client will not find the data item in the local database. If the data item is not (830) found in the local database, the client checks to see if there is a network connection. If there is (816) a network connection, the client sends a request to the server system for the data item, and receives responses as described in greater detail above. For example, where the web application is a web-based email application, the first time that the web application needs an email conversation (e.g., to display the conversation), it must request the conversation from the web-based email server. In this example, the web-based email server computes a checksum on the conversation and sends a unique identifier associated with the conversation and the checksum to the client, which are stored in a local database. Then, the second time the web application needs the conversation, it can simply send the unique identifier associated with the conversation and a checksum to the server.

If the data item is (832) found in the local database, the client retrieves the data item from the local database, including the first checksum (or the client-computed checksum) and a unique identifier associated with the data item. The client sends (836) a request to the server system. In some embodiments, this request includes the first checksum (or the client-computed checksum). In some embodiments, the client device periodically sends the first checksum and the unique identifier to the server system, where it is compared with a checksum for the data item on the server. For example, a web-based email application can periodically confirm that a particular conversation is up to date by sending a checksum of the conversation to the web-based email server. In this example, the server computes a checksum of the conversation (as stored in the server system) and, if the checksums do not match, sends the conversation (as stored in the server system) to the web-based email application.

In some embodiments the data item has a plurality of discrete components (e.g., the data item is a conversation and the discrete components are messages) and the request includes (838) identifiers of components (e.g., the request includes message identifiers when the data item is a conversation that is a list of messages). In some embodiments the identifiers of components include all of the components associated with the data item on the client (e.g., messages identifiers for all of the messages in the conversation, as stored on the client).

In some embodiments, the request (e.g., 836) sent by the client to the server system is automatically generated by the web application. In some embodiments the automatically generated request is to preload the data items that are most likely to be accessed by the user. In one example, an inbox is displayed containing header information (e.g., the sender and subject) for a plurality of email messages or groups of email messages (e.g., one or more related email messages may be grouped together under one header as a single conversation) that are associated with an attribute “inbox.” In this example, the web application automatically generates a request for the data item associated with each of the headers in the current view of the inbox. It should be understood that the current view of the inbox could be any one of: all of the headers in the inbox (e.g., all of the messages/conversations in the inbox are requested), all of the headers that are included on a current page of the inbox (e.g., all of the messages/conversations that the user can scroll to on the screen are requested), and all of the headers that are currently displayed on the display (e.g., only the messages/conversations associated with the headers that are currently viewable by the user without scrolling are requested).

In some embodiments data items may be displayed as a compressed representation (e.g., in a compressed form) or as an uncompressed representation (e.g., in an uncompressed form). Data items displayed in a compressed form include less information than data items displayed in an uncompressed form. In some embodiments, a data item displayed in a compressed form includes only the header information (e.g., subject, sender and date/time) about the data item (e.g., the header information about the first message in a conversation or the first conversation in a thread list). In some embodiments, a data item displayed in a compressed form includes displaying header information for a plurality of the components of the data item. For example, when the data item is a thread list (e.g., an inbox), displaying the thread list in a compress form includes displaying headers for one or more of the components (e.g., conversations) of the thread list. For example, when the data item is a conversation (e.g., a conversation), the displaying the conversation in a compressed form includes displaying headers for one or more of the components (e.g., messages) of the conversation.

In some embodiments, the web application initially only has enough information about a data item to display the compressed form of the data item (e.g., initially the headers of all of the most recent conversations are downloaded from the server). When the web application requests access to a particular data item (e.g., the thread list such as an email inbox, including one or more conversations), the compressed form of the data item is displayed (e.g., the headers of all of the conversations in the inbox are displayed). In addition, when the web application requests access to a particular data item (e.g., a web email inbox), the full data item is downloaded so that the web application can display the uncompressed form of the data item (e.g., the headers or content of all of messages in each of the conversations). For another example, the web application may initially display a conversation in a compressed form (e.g., only displaying the headers of one or more of the email messages in the conversation), and may display the conversation in an uncompressed form (e.g., displaying full text of one or more additional email messages in the conversation) upon receiving a request from a user to display the uncompressed form of the conversation (e.g., a request to view the full text of one of the messages in the conversation).

In some embodiments, after the request has been sent to the server system, the client performs (840) an operation on the data item (e.g., displaying the data item) that is stored in the local database. In some embodiments, this operation is performed while waiting for a response from the server system.

In some embodiments, the server system receives (842) the second request for the data item from the web application, wherein the second request includes an identifier of the data item and the first checksum. In some embodiments the data item has a plurality of discrete components (e.g., the data item is a conversation and the discrete components are messages) and the request includes (843) identifiers of components (e.g., the request includes message identifiers when the data item is a conversation that is a list of messages). In response, the server system retrieves the data item from the central database (844). In some embodiments, the server system has operations to perform on the data item (e.g., operations that were stored in write queue and sent with the request). The server system performs (846) any operations modifying the data item (e.g., performs all operations from the write queue, as discussed in greater detail above with reference to FIGS. 7A-7B), and then computes (848) a second checksum of the data item.

In some embodiments the server system compares the first checksum (or the client-computed checksum) to the second checksum. If the checksums match (850), then the server system determines that the data item has not been updated, and sends (852) a reply to the client device indicating that the data item has not been updated. In some embodiments, the reply indicating that the data item has not been updated is an empty reply (e.g., it is substantially similar to a reply indicating that the data item has been updated, except that it does not contain any data to add to the data item or replace the data item in the local database). The client receives (854) the response from the server, and because there is no (856) replacement data, the process ends (857).

If the checksums do not match (858), then the server system determines that the data item has been updated and sends (860) a reply to the client indicating that the data item has been updated. It should be understood that there are multiple strategies for updating a data item in the local database: in accordance with some embodiments, either 1) all of the data associated with the data item is replaced, or 2) just the components of the data item that have been changed or are new are replaced/added to the data item.

In some embodiments, the reply includes an updated data item (862) and the second checksum (e.g., the entire updated data item is sent to the client). The client receives (854) the response from the server, and because there is (864) replacement data (e.g., a replacement conversation), the data item and the second checksum received from the server system are stored (866) in the local database, and replace the old data item (e.g., the previously stored conversation) and the first checksum (or the client-computed checksum). This embodiment is particularly beneficial where network latency is high (e.g., communication takes a long time) and the network connection has a high bandwidth (e.g., large amounts of data can be transferred easily).

In some embodiments, after the updated data item (or data item components) are received from the server system, the data item is redisplayed (867) in the web application. For example, a user may submit a request to view a conversation (e.g., a list of email messages) in an email inbox. In this example, the web application initially displays the copy of the conversation that is stored in the local database. In conjunction with displaying the locally-stored conversation, the web application simultaneously (or near simultaneously) sends a request to the server system including a checksum locally-stored conversation. In this example, if the server system sends a replacement conversation (update data item) or additional email messages for the conversation (new/updated components), then the web application redisplays the updated conversation (or the new emails in the conversation). In some embodiments, the data item is automatically updated (e.g., automatically redisplayed). In some embodiments, a message indicating that the currently displayed data item is not up to date (e.g., that there is an updated data item available) is displayed. For example, the message could be a message such as: “new messages have been received for this conversation, would you like to display them?” In some embodiments, an updated data item is displayed (e.g., the data item is redisplayed) when the user requests that the updated data item be displayed (e.g., by selecting a “reload” button or by selecting a “display new messages” button).

In some embodiments, where the request from the client included component identifiers (e.g., message identifiers) the reply includes (868) one or more new components. In some embodiments a new component of a data item is a component with a component identifier that does not match any of the identifiers of the components of the old data item (e.g., a new email has been added to the conversation). In some embodiments a new component is an updated component that has a different checksum than the old component with the same component identifier (e.g., an email in the conversation has been modified). New components are components that are associated with the data item, but are not associated with any of the component identifiers included in the request.

For example, where the data item is a conversation and the components are email messages in a conversation, the client sends the server a request including the identifier of the conversation and a list of email messages (e.g., a first email, a second email, and a third email) that are components of the conversation. In this example, the server retrieves the conversation from the central database and notes the message identifiers included in the conversation (e.g., a first email, a second email, and a third email, a fourth email and a fifth email), and sends only the new email messages (e.g., the fourth email and the fifth email) to the client device in the reply. The client receives (854) the response from the server, and because there is (864) replacement data, the new components of the data item are stored (866) in the local database and associated with the data item in the local database. Additionally, the second checksum replaces the first checksum (or the client-computed checksum) in the local database. This embodiment is particularly beneficial where network latency is high (e.g., communication takes a long time) and the network connection has a lower bandwidth (e.g., there are some limitations on how much data can be transferred, so it is beneficial to send only a few components of the data item when only a few components are necessary to synchronize the data item stored in the local database with the data item stored in the central database).

In some embodiments, where the request from the client did not include component identifiers, the reply includes (870) a request for the component identifiers in the data item. In this embodiment the client receives (872) the request for component identifiers associated with the data item (e.g., message identifiers associated with a conversation or thread identifiers associated with a thread list). The client retrieves (874) the component identifiers associated with the data item from the local database and sends (876) the component identifiers (along with the associated unique identifier for the data item) to the server system. The server system receives the request and retrieves (878) the data item indicated by the unique identifier from the central database.

The server system compares (880) the component identifiers associated with the data item in the central database with the component identifiers sent to the server system by the client device. By comparing the two sets of component identifiers, the server system determines (882) which components, if any, of the data item in the central database are new. In one example, the data item is a conversation with three messages (e.g., components) where the client's version of the conversation has only the first two messages, while the server system's version of the conversation has all three messages. In this example, the client sends the server system message identifiers for a first message and a second message in the conversation. In this example, the server system retrieves the conversation from the central database, identifies the first message, the second message and a third message. In this example, the server system compares the messages identifiers from the client to the messages retrieved from the central database and determines that the third message is a new message (e.g., a new component).

In some embodiments, after determining that there are one or more new components in the data item, the server system sends (884) the one or more new components to the client with the second checksum. The client receives (886) the one or more new component(s) and the second checksum, and stores (888) the one or more new components and the second checksum in the local database. It should be noted that while this embodiment includes more trips back and forth, overall the trips include a smaller quantity of data transferred between the client and server. Thus, this embodiment is particularly beneficial where network latency is low (e.g., communication takes a short time) and the network connection has a low bandwidth (e.g., there are limitations on how much data can be transferred, so it is beneficial to send only those components of the data item that it is necessary to send).

In some embodiments a combination of the methods (e.g., updating the whole data item, sending components or sending a request for a list of components and then sending the components) for updating data items are used, the client device determines the bandwidth of the connection between the client device and the server system and/or the latency of the connection and determines the best method to use based on considerations of bandwidth availability or lag time due to network latency. In some embodiments, the server system determines the bandwidth of the connection between the client device and the server system and/or the latency of the connection and determines the best method to use based on considerations of bandwidth availability or lag time due to network latency.

Each of the methods described herein may be governed by instructions that are stored in a computer readable storage medium and that are executed by one or more processors of one or more server systems 106 or client devices 102. Each of the operations shown in FIGS. 7A-7B and 8A-8D may correspond to instructions stored in a computer memory or computer readable storage medium.

The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 

What is claimed is:
 1. A client device comprising: one or more processors; memory; and one or more programs, wherein the one or more programs are stored in the memory for execution by the one or more processors, the one or more programs including instructions for: receiving a request to perform an operation on a respective data item associated with a web application, wherein data items associated with the web application are accessible via a network connection between the client device and a server system; in response to the request to perform the operation on the respective data item, without regard to an availability of the network connection between the client device and the server system, determining whether the respective data item is stored in a local database in memory of the client device; sending, to the server system, a request for the respective data item, the sending being responsive to a determination that the respective data item is not stored in the local database; and performing the operation on the respective data item stored in the local database, the performing being responsive to a determination that the respective data item is stored in the local database.
 2. The client device of claim 1, wherein the local database is stored in non-volatile memory.
 3. The client device of claim 1, wherein the web application is a web-based email application with offline capabilities.
 4. The client device of claim 3, wherein the data items are thread lists or conversations in the email application.
 5. The client device of claim 1, wherein when the client device detects that the local database is full, at least some data items in the local database are marked for deletion in accordance with a cache replacement policy.
 6. The client device of claim 1, further comprising instructions for: in accordance with a determination that the operation involves updating the respective data item, writing information characterizing the operation to a write queue for the web application along with the respective identifier of the respective data item so as to enable the operation to also be performed on a copy of the respective data item stored at the server system; and when a network connection exits between the client device and the server system after performing the operation on the respective data item, draining the write queue to the server system.
 7. The client device of claim 6, wherein the write queue includes a plurality of operations for corresponding data items, and draining the write queue includes sending the operations in the write queue to the server system in the order in which they were stored in the write queue.
 8. The client device of claim 1, wherein said determining whether the respective data item is in a local database in memory of the client device occurs while the client device has a network connection to the server system.
 9. The client device of claim 1, wherein the local database contains a plurality of data items, each data item of the plurality of data items being associated with a corresponding identifier that enables the corresponding data item to be uniquely identified on the client device and on the server system.
 10. A method, comprising: performing by a client device, including one or more processors and memory storing one or more programs, the method comprising the following steps: receiving a request to perform an operation on a respective data item associated with a web application, wherein data items associated with the web application are accessible via a network connection between the client device and a server system; in response to the request to perform the operation on the respective data item, without regard to an availability of the network connection between the client device and the server system, determining whether the respective data item is stored in a local database in memory of the client device, sending, to the server system, a request for the respective data item, the sending being responsive to a determination that the respective data item is not stored in the local database; and performing the operation on the respective data item stored in the local database, the performing being responsive to a determination that the respective data item is stored in the local database.
 11. The method of claim 10, wherein the local database is stored in non-volatile memory.
 12. The method of claim 10, wherein the web application is a web-based email application with offline capabilities.
 13. The method of claim 12, wherein the data items are thread lists or conversations in the email application.
 14. The method of claim 10, wherein when the client device detects that the local database is full, at least some data items in the local database are marked for deletion in accordance with a cache replacement policy.
 15. The method of claim 10, further comprising: in accordance with a determination that the operation involves updating the respective data item, writing information characterizing the operation to a write queue for the web application along with the respective identifier of the respective data item so as to enable the operation to also be performed on a copy of the respective data item stored at the server system; and when a network connection exits between the client device and the server system after performing the operation on the respective data item, draining the write queue to the server system.
 16. The method of claim 15, wherein the write queue includes a plurality of operations for corresponding data items, and draining the write queue includes sending the operations in the write queue to the server system in the order in which they were stored in the write queue.
 17. The method of claim 10, wherein said determining whether the respective data item is in a local database in memory of the client device occurs while the client device has a network connection to the server system.
 18. The method of claim 10, wherein the local database contains a plurality of data items, each data item of the plurality of data items being associated with a corresponding identifier that enables the corresponding data item to be uniquely identified on the client device and on the server system.
 19. A non-transitory computer readable storage medium storing one or more programs, the one or more programs comprising instructions, which when executed by a client device with one or more processors, cause the client device to: receive a request to perform an operation on a respective data item associated with a web application, wherein data items associated with the web application are accessible via a network connection between the client device and a server system; in response to the request to perform the operation on the respective data item, without regard to an availability of the network connection between the client device and the server system, determine whether the respective data item is stored in a local database in memory of the client device send, to the server system, a request for the respective data item, the sending being responsive to a determination that the respective data item is not stored in the local database; and perform the operation on the respective data item stored in the local database, the performing being responsive to a determination that the respective data item is stored in the local database.
 20. The computer readable storage medium of claim 19, wherein the local database is stored in non-volatile memory.
 21. The computer readable storage medium of claim 19, wherein the web application is a web-based email application with offline capabilities.
 22. The computer readable storage medium of claim 21, wherein the data items are thread lists or conversations in the email application.
 23. The computer readable storage medium of claim 19, when the client device detects that the local database is full, at least some data items in the local database are marked for deletion in accordance with a cache replacement policy.
 24. The computer readable storage medium of claim 19, further comprising instructions to: in accordance with a determination that the operation involves updating the respective data item, write information characterizing the operation to a write queue for the web application along with the respective identifier of the respective data item so as to enable the operation to also be performed on a copy of the respective data item stored at the server system; and when a network connection exits between the client device and the server system after performing the operation on the respective data item, drain the write queue to the server system.
 25. The computer readable storage medium of claim 24, wherein the write queue includes a plurality of operations for corresponding data items, and draining the write queue includes sending the operations in the write queue to the server system in the order in which they were stored in the write queue.
 26. The computer readable storage medium of claim 19, wherein said determining whether the respective data item is in a local database in memory of the client device occurs while the client device has a network connection to the server system.
 27. The computer readable storage medium of claim 19, wherein the local database contains a plurality of data items, each data item of the plurality of data items being associated with a corresponding identifier that enables the corresponding data item to be uniquely identified on the client device and on the server system. 